The present invention relates to a method for controlling at least one hydraulic cylinder in a work machine and a control system for a work machine.
The invention will be described below in connection with a work machine in the form of a wheel loader. This is a preferred but in no way limiting application of the invention. The invention can also be used for other types of work machines (or work vehicles), such as an excavator loader (backhoe) and excavating machine.
The invention relates, for example, to controlling lifting and/or tilting cylinders for operating an implement.
It is desirable to provide a method for controlling a hydraulic cylinder, preferably for a lift function and/or tilt function, that provides smooth operation.
According to an aspect of the present invention, a method is provided for controlling a hydraulic cylinder in a work machine, which hydraulic cylinder is arranged to move an implement that is subjected to a load, with the hydraulic cylinder being controlled by a hydraulic machine, comprising the steps of detecting that a lifting movement of the implement is to be initiated, and attaining a basic speed of the hydraulic machine before lifting takes place. This control method provides a reduction in the starting friction in a hydraulic machine (pump) at the commencement of a lifting movement.
According to a preferred example, the method comprises the steps of the hydraulic machine attaining the basic speed by draining the port of the hydraulic machine that is connected to the piston side of the hydraulic cylinder and thereby allowing a certain amount of leakage flow from the hydraulic machine at the commencement of the lifting movement. A communication path is preferably established between the port of the hydraulic machine that is connected to the piston side of the hydraulic cylinder and a tank, thereby allowing a certain amount of leakage flow from the hydraulic machine to the tank at the commencement of the lifting movement. It is, however, not necessary to drain the port of the hydraulic machine to the tank. According to an alternative, the port of the hydraulic machine that is connected to the piston side of the hydraulic cylinder can be connected to a second port of the hydraulic machine that forms an inlet to the hydraulic machine.
According to a specific example, the method comprises the steps of achieving said draining by opening a control means on a line that is connected to the port of the hydraulic machine.
It is desirable to achieve a control system, preferably for a lift function and/or tilt function, that provides smooth operation
According to an aspect of the present invention, a control system is provided for a work machine comprising a hydraulic machine and at least one hydraulic cylinder, characterized in that a first port of the hydraulic machine is connected to a piston side of the hydraulic cylinder via a first line, and in that a control means is arranged to achieve a draining from the first port of the hydraulic machine in order to allow a certain amount of leakage flow from the hydraulic machine at the commencement of a lifting movement.
Said control means preferably comprises an electrically controlled valve. The valve is preferably continuously variable, but an on/off valve is also possible.
The hydraulic cylinder is preferably adapted to move an implement in order to perform a work function. According to a first example, the hydraulic cylinder comprises a lifting cylinder for moving a load arm which is pivotably connected to a vehicle frame, the implement being arranged on the load arm. According to a second example, the hydraulic cylinder comprises a tilting cylinder for moving the implement which is pivotably connected to the load arm.
Further preferred embodiments and advantages of the invention emerge from the following description.